Allow bbcode image URLs to use style properties

Currently, a bbcode editor image URL is a fixed url which can't vary by theme without a lot of wonky trickery. Being able to vary the icons per theme vastly helps for handling themes with very different color coding.

Being able to refer to style properties in the admin-definable URLs ie; @imagePath would help with this.

I'm fairly sure this will be a breaking change for XF1.x since the URL may be inspectable by other code which expect it to be a pure URL.